1. Field of the Invention
The present invention relates to a TFT ion sensor constituted with TFT (Thin Film Transistor), a measuring method using the TFT ion sensor, and a TFT ion sensor apparatus using the TFT ion sensor.
2. Description of the Related Art
Recently, ion concentration sensors used for bio-sensing, for example, are utilized in the fields of medical services and the like. The ion concentration sensor is acquired by combining a hydrogen ion concentration detecting function on an inorganic oxide surface, an oxygen molecule identifying function on the inorganic oxide surface, and an electrochemical device.
An ion sensitive FET (FET ion sensor) using a silicon substrate MOSFET (Metal Oxide Semiconductor Field Effect Transistor) has already been developed into a product, and it is mainly utilized as a pH (potential of hydrogen) sensor of a solution. It is electrochemically known that electric double layers are essentially formed in the interface between a solution and an insulating film. The potential difference generated in the electric double layers changes depending on pH of the solution, and it is the measuring principle thereof to sense the value of pH by reading out the change amount as Vth (threshold voltage) shift of FET.
According to the Nernst theory of electrochemistry, a potential change of about 59 mV is generated in the electric double layers in the interface of pH solution/oxide film because pH of the solution changes by “1” (the hydrogen concentration changes by one digit) at 25° C. That is, 59 mV/pH is the theoretical limit of the measurement sensitivity. The sensitivity of the actual ion sensitive FET products is about 45 to 50 mV/pH, and it is being studied to bring the sensitivity close to the theoretical limit.
In association with bio-medical industries, a precise sensing technique of substances in various solutions will become important for the future. In bio-sensing, a method of detecting a change in the hydrogen ion concentration at last via an enzyme reaction or the like is used often. Thus, regarding pH, a technique capable of sensing a small pH change is being required. Therefore, there has been an increasing demand for the necessity for improving the measurement sensitivity (increasing the sensitivity to be higher than 59 mV/pH) by some kind of means.
Next, documents of related techniques associated with the present invention (referred to as “related techniques” hereinafter) will be described.
Related to the TFT ion sensor utilizing Vt shift of TFT, there is a report of a case by F. Yan, et al., in which a polycrystalline silicon TFT is used (“Applied Physics Letters”, vol. 86, 053901 (2005) (Non-Patent Document 1)). A silicon nitride film is used as an ion sensitive insulating film, pH sensing is performed, and the sensitivity of 54 mV/pH is acquired.
J.-C. Chou et al., performed pH sensing by using amorphous silicon TFT, and the sensitivity of 58 mV/pH was achieved (“Sensors and Actuators”, B62, 92-96 (2000) (Non-Patent Document 2)).
K. Koike et al., performed pH sensing by using a transistor that used a zinc oxide that was an oxide semiconductor as a semiconductor active layer, and the sensitivity of −20 μA/pH was acquired (“Japanese Journal of Applied Physics”, vol. 46, L865-L867 (2007) (Non-Patent Document 3)).
There is disclosed a method with which an ion sensitive insulating film is formed on an oxide semiconductor film in a bottom-gate type TFT using an oxide semiconductor, and the voltage of a reference electrode in a sensing solution is changed while a constant voltage is applied to a bottom gate electrode to perform sensing from the shift in the threshold voltage of the reference electrode voltage-drain current property (Japanese Unexamined Patent Publication 2012-73101 (Patent Document 1)).
There is disclosed a method with which an ion sensitive insulating film is formed by using a polysilazane solution in a bottom-gate type TFT having carbon nanotube as an active layer, and a top gate electrode is formed on the ion active insulating film to improve the property (Japanese Unexamined Patent Publication 2010-192599 (Patent Document 2).
There is disclosed a TFT sensor capable of performing sensing even in an ultraviolet atmosphere through cutting ultraviolet rays by a dual gate electrode of a dual-gate type TFT using an oxide semiconductor and forming an ion sensitive insulating film on the dual gate electrode (Japanese Unexamined Patent Publication 2013-76656 (Patent Document 3)).
As described above, the TFT ion sensors using the silicon semiconductor and the oxide semiconductor have been reported. All of those perform pH sensing from the shift in the property of “voltage applied to a reference electrode dipped in a sensing-target solution” with respect to “drain current”.
As described above, various techniques regarding the ion sensor using TFT have been disclosed. However, disclosed in all of those documents are the methods with which the voltage of the reference electrode dipped in the sensing-target solution is changed and the ion concentration is detected from the shift of the reference voltage electrode-drain current property. With such methods, 59 mV/pH is the theoretical limit of the sensitivity in pH sensing, for example, according to the Nernst theory. The theoretical limit is constant without depending on the semiconductor material and the ion sensitive insulating film.
The pH sensor using MOSFET has already been put into practical use, and those with the sensitivity of about 55 mV/pH which is close to the theoretical limit mentioned above have been made into products. Since the actual products have been and are being made, it is considered that the sensitivity of about 55 mV/pH is sufficient when sensing the hydrogen ion concentration itself in the solvent.
Further, in a case where a bio-sensing function is implemented by providing an enzyme which solidifies only a specific substance on the ion sensitive insulating film of the TFT ion sensor, still high sensitivity is required. For example, when detecting a hormone substance existing in a neutral solution (hydrogen ion concentration: 10−7 mol/L) in concentration of about 10−9 to 10−8 mol/L based on a change in the hydrogen ion concentration, it is required to detect the change in pH of about 0.01 to 0.1 with high precision. However, according to the Nernst theory limit, it is required to detect a small property shift of about 0.59 mV to 5.9 mV. Thus, due to the influences of measurement precision fluctuation and the like, it is difficult to perform precise detection. For example, considering the influences due to the measurement precision fluctuation and the like by heat, the sensitivity is decreased further than the theoretical limit. When the so-called original signal-to-noise ratio (S/N ratio) is decreased, the noise level is included and amplified when amplification is done by an external circuit or the like. Thus, practically, the sensitivity cannot be increased.
With the related technique that is bound to the sensitivity limit according to the Nernst theory, it is difficult to detect a small amount of concentration substance such as hormone with high precision.
It is therefore an exemplary object of the present invention to provide a TFT ion sensor capable of sensing a small ion concentration change with high sensitivity.